In drop on demand inkjet printing, ink is ejected through a small orifice by either a thermally induced bubble or a mechanical pulse. In continuous inkjet printing processes, ink drops are continuously ejected and their path controlled by charge induced deflection.
The ink droplets land on a support material, most frequently paper, and form an image. Balancing the droplet ejection efficiency, resistance to nozzle clogging, image quality and image durability is complicated and nearly always requires compromise. Image durability is an important attribute, especially for images used for display purposes which may require cleaning. Another important attribute is the need to control gloss of the printed areas.
Several approaches to improving inkjet image durability and gloss have been disclosed. For example, film-forming solution or latex polymers are often contemplated as ink addenda in order to impart glossiness and durability to printed images. In U.S. Pat. No. 5,607,999 (Seiko Epson Corporation), dye based inks containing water soluble polymers such as poly(vinyl pyrrolidone) and acrylic acid based copolymers are disclosed. These inks are printed using a piezoelectric pulse-generating printhead in which the energy imparted to the ink is entirely mechanical. However, such inks would be unacceptable for more general use in thermal print heads. Due to the nature of these inks to form films, particularly at high temperatures, in thermal inkjet printing or at the air interface near the orifice, nozzle clogging often results.
U.S. Pat No. 5,415,686 (Canon Kabushiki Kaisha) discloses the addition to inks of particulate cellulose, ranging in size from 10 nm to 10 microns, in order to improve image quality and dye fixability on plain paper. Such particles are not intended to be fused after printing, and in fact could not fuse under typical lamination or fusing temperatures as cellulose does not exhibit a glass transition temperature, but decomposes at temperatures in excess of 200.degree. C.
Film forming solution polymers such as those described above require fairly high molecular weights in order to create a printed layer resistant to cohesive failure. Low molecular weight polymers are often added to dye- and pigment-based inks in order to function as humectants and dispersants. For example, U.S. Pat. No. 5,519,085 (E.I. DuPont de Nemours and Company) discloses the addition of ABC triblock copolymers which bind to pigmented colorants and improve ink stability. However, such additives are generally of low molecular weight or low glass transition temperature such that they do not possess sufficiently robust mechanical properties to form a rub-resistant film.
Another approach to yield waterfast inkjet images is disclosed in U.S. Pat. No. 4,692,188 (Xerox Corporation) in which organic solvent-soluble dyes are entrapped in polymeric particles. The polymer-dye particles range in size from 300 to 1000 nm, a size range which tends to cause nozzle plugging, particularly when forming the very small drop volumes preferred for high quality image formation. The preparation of such dispersions entails several steps involving handling of environmentally objectionable solvents. In addition, the application of such an approach is not extended to pigmented ink systems and furthermore, fusing of resultant printed images is not mentioned.
Another approach to utilizing polymeric beads in inks is outlined in U.S. Pat. No. 5,100,471 (Xerox Corporation). In this case, the ink colorant comprises a polymeric core with a silica shell on which dyes have been covalently bonded. While such an image is anticipated to be waterfast, it could not be successfully fused to a more abrasion-resistant state as the silica shell would interfere with any fusing process.
Another approach to improving printed ink durability and appearance is printing an image on a support material comprising particulate thermoplastic species, then fusing the image at elevated temperature and pressure. Such an approach has been disclosed for both dye based and pigment based inkjet inks. For example, in U.S. Pat. No. 5,764,262, (E.I.DuPont de Nemours and Company), a "durable image is formed by printing a pigmented aqueous ink onto a substrate bearing a hydrophilic thermoplastic polymer having cross-linking groups, then heating the printed image to encapsulate the pigment and cross-link the polymer." It would be preferred to provide a receiver without the processing disadvantages of cross-linking chemistries and without the need to encapsulate the pigment.
In another example, U.S. Pat. No. 5,374,475 (Celfa AG), a recording layer useful for inkjet printing is disclosed which comprises a microporous layer which may then be eliminated by heat and pressure. However, if such approaches are followed, only specially coated paper may be used to generate acceptable images.
In order to attain more freedom in the choice of printing media, it would be desirable to include melt-fusible polymers in the inks.